Yes I have done a water only fast, it wasn’t voluntary though. I had bad strep throat, and glandular fever at the same time. I was only getting about 8 to 12 ounces of water a day for about 32 days. It took me a week just to start eating solid food.
Depends on why you’re fasting. Technically the calories in mct oil would be a break in a fast, but as it’s basically equivalent to a ketone it doesn’t break the fast and bring you out of ketosis. Same with the apple cider vinegar, lemon juice and black coffee, nearly zero calories.
Yes it is important what and how much you consume after a fast. If you start eating sugar and refined carbs, your inflammatory response will be through the roof, and as I said you need to eat pre and pro biotic foods to get your gut bacteria and stomach working properly. The gut lining also reduces its ability to absorb nutrients after 48 hours, so you need smaller meals to start, or you can overload your stomach and you will get dioreah.